Four local isolates of Metarhizium anisopliae (Bp, Mo, Ma and Ml) were bioassayed against larvae of Oryctes rhinoceros in the laboratory. The larvae were surface-sterilized in 0.3% sodium hypochlorite and dipped into spore suspensions containing 108 conidia ml-1. Of the four isolates, two long-spored isolates (Bp and Mo) were highly pathogenic against the beetles. Both isolates caused 100% mortality within 12 days of treatment and caused 71% to 75% mycosis. The times required to kill 50% (LT50) of Oryctes larvae for isolates Bp (LT50 = 9.1 days) and Mo (LT50 = 8.9 days) were shorter than short-spored isolates Ma (LT50) = 10.0 days) and Ml (LT50 = 14.0 days). The mortality rates (regression slopes) showed that isolate Bp (5.32) killed Oryctes larvae slightly faster than isolate Mo (5.17); therefore, isolate Bp was selected for a field experiment.

A field trial was conducted in seven-month-old heaps of chipped oil palm trunks, using a single application of wet and dry inoculum. Using wet inoculum, each plot was drenched separately with 10 litres plot-1 of water containing 108, 109 and 1010 conidia. For dry inoculum, the plots were broadcasted with inoculated maize at 3 and 6 kg plot-1. In the field, Metarhizium infects adults, pupae and all larval stages of Oryctes beetles. At three months after treatment, pooled mortality for all stages of Oryctes beetles at the highest application rate was between 37% (dry inoculum) and 51% (wet inoculum). A great impact of Metarhizium was found only on third instar larvae, where it was able to reduce the number to about 5.5-5.8 larvae / sampling plot. This was significantly lower than the control which had 34.8 larvae / sampling plot.

The densities of viable spores in plots treated with wet inoculum at 1010 conidia (158.6 cfu) and dry at 6 kg (169.0 cfu) inoculum were significantly higher than the other application rates, especially at 3-month of treatment. A higher density of viable spores in breeding materials enhanced the disease development and subsequently reduced the beetle population. Wet inoculum had more advantages over dry inoculum had more advantages over dry inoculum. It was easily prepared using water from readily available sources such as field drains, river and extin mining pools. This study showed that wet inoculum was effective for control of Oryctes beetle and was more economical to apply than dry inoculum.